Although it is generally believed that serum and liver lipid levels increase with age we have observed that this situation differs with rat strain. Wistar or Sprague-Dawley rats show increased lipids with age, Fisher rats do not. We propose to identify the strain differences and identify the underlying physiological causes. Lipid levels will be related to age and weight. We will also investigate serum and liver sterol and fatty acid spectra as well as distribution of triglycerides and phospholipids. We also propose to measure: cholesterol biosynthesis from acetate mevalonate and octanoate; cholesterol degradation (7 hydroxylase); bile acid pool size; cholesterol absorption; and lipoprotein synthesis and metabolism. Our work will yield a synthesis of all the changes in lipid metabolism wich occur in the aging rat and may explain why in some strains serum and liver lipids rise with age whereas in others they do not.